Infantile hemangiomas are the most common tumors in infancy, typically appearing on head and neck around the second week of life, growing rapidly (proliferating phase) over a few weeks and months, and slowly regressing over 7-10 years (involuting phase). Most hemangiomas are single, small lesions, but some can destroy normal tissue or threaten life. In studies of tissues and endothelial cells from proliferating-phase hemangiomas we have found expression of VEGFR1 to be only 10-20% of that in control endothelial cells and tissues. Low VEGFR1 levels result in VEGF-dependent activation of VEGFR2 and its downstream signaling targets, including genes already known to be abnormally expressed in hemangioma tissue. Addition of soluble VEGFR1 or VEGF antibodies to hemangioma endothelial cells reduces their high VEGFR2 signaling and proliferative activities. We have further shown that low expression of VEGFR1 in hemangioma is caused by reduced activity of an NFAT-controlling complex involving VEGFR2, the integrin- like receptor TEM8 and pi integrin. In three hemangioma patients (of nine studied) heterozygous missense mutations in VEGFR2 or TEM8 provide an explanation for the reduced activity of the VEGFR2/TEM8/pi integrin complex. Future studies aim at generating mice carrying TEM8 and VEGFR2 mutations for studies of the effects of the mutations on angiogenesis and identification of additional components of the VEGFR2/TEM8/pi integrin -containing complex. In collaboration with Project 3 such components will be screened for mutations in hemangiomas where mutations in VEGFR2 or TEM8 have not been found. Based on preliminary studies of pathways that regulate apoptosis in endothelial cells, we also plan studies of involuting hemangiomas aimed at identifying strategies to accelerate involution in clinically problematic tumors. In collaboration with Project 2 transplantation into immunocompromised mice will be used for preclinical testing of disease-modifying drugs and to test the hypothesis that the hemangioma endothelial phenotype can be induced in cells carrying risk factor mutations by localized and sustained activation of VEGFR2. RELEVANCE (See instructions): These studies are anticipated to lead to identification of targets for drugs to effectively treat rapidly growing infantile hemangiomas, the most common tumors of childhood. The work is also likely to have impact on other diseases in adults involving abnormal angiogenesis.